Vita

Vita in Mahou Shoujo Lyrical Nanoha A's Vita enters A's on a hunt with Zafila for strong Linker Cores to absorb into the Book of Darkness. She targets Nanoha Takamachi, which she deems to be a juicy target, unlike the usual TSAB "redshirts" which can barely fill up a single page of the Book of Darkness. Nanoha puts up a decent fight, but Vita tilts the deck with two cartridges (bullet shaped things that contain high-pressure magic) manages to overpower her, obtaining a power superiority so vast that she heavily damaged the Raising Heart. However, before she could get the Linker Core, Fate Testarossa and Yūno Scrya come to the rescue. Vita fights them off with her fellow Wolkenritter, Signum and Shamal, and they declare themselves enemies. The battle ends with Shamal finally managing to get Nanoha's Linker Core and Nanoha breaking Vita's barrier.

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Vita fights for the sake of protecting their mistress, Hayate, whom she is completely dedicated to and possibly (in the manga, certainly) in love with. The Wolkenritter attack mages to steal their Linker Cores, their sources of magic, to fill the 666 pages of the Book of Darkness with this power before it saps any more of Hayate's and she becomes fully paralyzed. They continue to obtain Linker Cores and fight off interference from the Time-Space Administration Bureau (TSAB), mainly Fate and Nanoha- whose name Vita purposely gets wrong to annoy her- while they try to keep Hayate from knowing that they are stealing magic for pages; she has told them that as long as she is mistress of the Book, she will not allow anyone to be harmed over it. The Wolkenritter, especially Vita, are determined to save Hayate even if it means going against her wishes. What the Wolkenritter don't know about Hayate is that the Book of Darkness, once completed, may stop sapping her Linker Core for pages, but that does not mean that it will stop hurting her or undo its damage. In fact, the master of the Book dies upon its completion. Vita is the first to recognize that something may be wrong with what they are doing and that they have forgotten something very important, but her devotion to Hayate keeps her fighting for Linker Cores regardless. Just before the Book of Darkness is completed, Suzuka brings Nanoha, Arisa, and Fate to visit Hayate in the hospital while the Wolkenritter are there. The warring groups recognize each other and their shared wish at once, and meet on the roof of the hospital. Nanoha and Fate try to tell the Wolkenritter about what will happen to Hayate, but their confrontation is cut short by both the interference of two masked men and the completion of the Book of Darkness with the Linker Cores of the Wolkenritter themselves. The masked men are actually women, Alia and Lotte Liese, who frame Nanoha and Fate for the death of Vita and the others. Hayate screams in horror for all of this to stop and is taken over by the will of the Book of Darkness, becoming the fearsome, possessed woman inside of it just as the real duo escapes their prison. Meanwhile, Vita's spirit is trapped inside the Book of Darkness with the others. When Hayate rejects her own wish of vengeance, she reprograms the entire Book, but a virus still remains inside, the one that caused the evil aspect of the Book in the first place. Hayate awakens the spirit of Vita and her friends inside the book of Darkness, and she decides to join the fight against the automated defence program that Hayate describes as "the 'darkness' in the Book of Darkness." Vita finally recognizes Nanoha as an equal, though she won't admit it, and calls her by her real name. In an epilogue taking place six years later, Vita has not aged, considering her nature as a computer program, and still lives with the other Wolkenritter and Hayate, posing as a relative.

Vita in Mahou Shoujo Lyrical Nanoha StrikerS Four years after the A's epilogue, Second Lieutenant Vita is sub-commander of the Stars Squad in the TSAB, working directly under Nanoha, who, in turn, obeys Hayate. She and the other Wolkenritter were kept close to Hayate and each other-- Shamal and Zafira are support, Signum is with the Lightning Squad-- because they work most effectively as a team. (Source: Wiki)